Commercially pure titanium (Ti) and Ti alloys are the metals of choice in orthopedic and dental applications because of their biocompatibility, resistance to corrosion and good mechanical properties such as lightweight, durability, high strength, and the ability to be prepared in different forms, shapes and textures. However, Ti or Ti alloy do not directly bond or attach to bone. Instead, there is a layer of fibrous tissue at the Ti-bone interface causing a weak interface that could increase the possibility of the implant loosening over a long period of time. On the other hand, materials such as calcium phosphate materials (e.g., hydroxyapatite, HA; tricalcium phosphate, β-TCP; and biphasic calcium phosphate, BCP), are bioactive, forming a direct bond and a uniquely strong interface with bone, but are not strong enough for load-bearing areas.
Commercial dental and orthopedic implants coated with plasma-sprayed HA were developed to combine the strength and superior mechanical properties of the metal (Ti or Ti alloy) and the bioactivity and osteoconductivity of the Ca—P compounds. Better bonding and fixation between implant and host tissue minimize the micro-movements that promote fibrous tissues formation at the implant/tissue interface and may cause implant failure. Coating with Ca—P materials was also shown to inhibit the release of substrate metal ions (Ti, Al, V) from substrate, ions that may be potentially harmful to cells and/or may interfere with the biomineralization process. There are, however, some shortcomings of the plasma-sprayed HA coating. Xray diffraction analyses of plasma-sprayed HA dental and orthopedic implants showed variable coating composition and surface morphology. The coatings were shown e.g., to consist of crystalline (principally HA) and non-crystalline (amorphous calcium phosphate, ACP) phases. The HA/ACP ratio in the coating varied from 30/70 to 70/30. [LeGeros R Z, LeGeros J P, Kim Y, Kijkowska R, Zheng R, Bautista C, Wong J L. Calcium phosphates in plasma-sprayed HA coatings. Ceramic Trans 48:173-189, 1995. LeGeros R Z, Kim YE, Kijkowska R, Zurita V, Bleiwas C, Yuang P-Y, Edwards B, Dimaano F, LeGeros J P. HA/ACP ratios in calcium phosphate coatings on dental and orthopedic implants: Effect on properties. Bioceramics 11. Singapore:World Scientific Publishing Co., pp 181-184, 1998.] The coating composition also differed from the coating layer closest to and away from the metal substrate and was also affected by the geometry of the implant. [LeGeros J P, Huang P Y, LeGeros R Z, Wong J L. Effect of substrate geometry on heat capacity and crystallinity on plasma-sprayed HA coatings. J Dent Res 1998; 77:2682.] ACP is much more soluble than HA, therefore the dissolution (or biodegradation) of the coating is very much affected by the HA/ACP ratio in the coating: the lower the ratio, the greater the rate of biodegradation. Some additional phases such as β-tricalcium phosphate (β-TCP), α-TCP, tetracalcium phosphate (TeTCP), and sometimes calcium oxide are also formed during the high temperature process of plasma spraying. Like ACP, these additional phases have higher solubility than HA. The uneven biodegradation of coating can result in a non-homogenous bone bonding or bone growth around the implant and/or delamination and separation of big fragments or debris from the coating materials that can cause premature disintegration of the coating—and severe complication in the osseointegration of the implant.
Osteogenic macromolecules (e.g., bone morphogenetic proteins (BMPs), bioactive peptides or proteins, and bone growth factors) have been shown to improve and increase the extent of bone formation. Coating on metallic implant can be also used as an effective carrier to deliver and release the osteogenic molecules to the site of implantation. To allow controlled release of bioactive molecules, their incorporation must be achieved during the coating procedure which requires physiological temperatures (37° C.). The incompatibility of the extremely high temperatures (about 30,000° C.) associated with the plasma spray method is obvious. In addition, the plasma-spray method, being a line-of sight technique,—will not provide uniform coating on complex shaped materials with internal cavities or macroporosities.
Because of the above-mentioned disadvantages of the plasma spray method, the potential of in situ Ca—P coating methods are being extensively explored. Three principal low-temperature coating methods are: (i) chemical deposition of Ca—P compounds by immersion of Ti or Ti alloy substrate in a calcifying solution containing Ca and P ions (chemical or biomimetic deposition); (ii) formation of Ca—P layer on substrates using sol-gel processing; and (iii) Ca—P coating using electrodeposition (ECD) method. Most of the coatings obtained by these methods do not give the shear and tensile strength comparable to that obtained by the plasma-spray method. Recently too, one of the present inventors and her colleagues reported a pulse-modulated ECD method of depositing octacalcium phosphate [Lin S, LeGeros R Z, LeGEros J P. Adherent octacalcium phospahte coating on titanium alloy using modulated electrochemical deposition method. J Biomed Mater Res 66A:819-828, 2003.] and calcium-deficient, carbonate-substituted and fluoride substituted apatite coatings with strength comparable to that obtained with the plasma-spray method [LeGeros J P, Lin S, LeGeros R Z. Electrochemically deposited calcium phosphate coating on titanium alloy. J Dent Res 79:560, 2000 (abstr no. 560.].
In order to enhance the adhesion and the coverage of Ca—P coating, several studies explored chemical and/or mechanical pre-treatment of the Ti or Ti alloy surfaces. Formation of bioactive TiO2 hydrogel layer has been shown to improve the nucleation of calcium phosphate during chemical deposition. TiO2 layer can be prepared by alkaline, H2O2, sol-gel or heat treatment methods. Kokubo and his collaborators [Wei M, Kim H M, Kokubo T, Evans J H. Optimising the bioactivity of alkaline-treated titanium alloy. Mat Sci Eng C-Bio S 20:125-134, 2002; Kim H M, Kokubo T, Fujibayashi S, Nishiguchi S, Nakamura T. Bioactive macroporous titanium surface layer on titanium substrate. J Biomed Mater Res 52:553-557, 2000. Takadama H, Kim H M, Kokubo T, Nakamura T. An X-ray photoelectron spectroscopy study of the process of apatite formation on bioactive titanium metal. J Biomed Mater Res 55:185-193, 2001.] demonstrated that the treatment of Ti with a NaOH solution followed by heat treatment at 600° C. forms a crystalline phase of sodium titanate layer on the Ti surface resulting in improved adhesion of apatite coating prepared by incubation in simulated body fluid (SBF). The authors concluded that release of the sodium ions from the sodium titanate layer causes formation of Ti—OH groups which react with the calcium ions from the SBF and form calcium titanate which then could act as nucleation sites for apatite crystal formation. Alkali treatment results in the formation of TiO2 layer leading to a negatively charged surface which in turn attracts cations such as calcium ions. Etching with acid followed by alkali treatment was also investigated to combine the surface roughness increase due to acid treatment and formation of TiO2 bioactive layer. TiO2 could be also prepared using H2O2 alone or mixture of acid/H2O2 or metal chlorides/H2O2 solutions.